MAN IN THE MODERN WORLD
so the size of the growth ring in its wood preserves for us the record of
the season. By measuring the growth rings of over two thousand big
trees, Douglass has given us a curve of climate which corresponds with
remarkable accuracy with what we have deduced from other sources.
Some of these trees date back four thousand years. In their trunks
we can read of the dry periods which spread civilization over the
world but spelled the ruin of the first Archaic Culture; of the "class-
ical" rainfall maximum, as Brooks calls it, which allowed Greece and
Rome and Yucatan to achieve their destiny; of the new drought
which brought the barbarians into the Holy City and raised the
Norsemen to their first height of activity. And they record for us the
final settling of the fertilizing, energy-giving belt of cyclonic weather
in its present place, a thousand miles and more northward of its old
Thus climatic belts have not shifted seriously for almost a thousand
years. What will happen to civilization when they move again we
can hardly foresee; but we cannot suppose that shifting climate will
respect our modern balance of power, any more than it spared the
civilizations of Mesopotamia. Climate is inexorable.
The question of the effects of climate and other natural phenomena
on human history is not all speculative. We can sec some of its very
practical ramifications in the problems of cattle, soil, and grasslands.
Here the chemistry of soils enters in as well as climate, but the two
are not without relation.
From time to time, in different parts of the world, cattle exhibit
perverted appetites, They take to chewing bones, and will sometimes
even devour the carcasses of other cattle that have died. These
abnormal instincts are invariably the prelude to grave disorders. Ixx
typical cases the bones grow soft, the joints become swollen, the
animals get thin and feeble and move stiffly and awkwardly; their
hoofs grow abnormally long; sterility and abortion are common.
Milch cows and young growing beasts are invariably the most seri-
ously affected; and imported modern breeds suffer worse than the
poorer native types. Sheep may be affected in the same sort of way;
and horses too, though more rarely.
These outbreaks, which may inflict severe losses, may only recur
every few years; or they may continue unabated for long periods.
In every case they are confined to particular regions. In such a
region, even in years when there is no actual disease, the arumals are